Is every electromagnetic radiation considered "light"? Somebody mentioned on Freenode chatroom for physics that All Electromagnetic Radiation are delivered in form of Photons not just light. 
Is it true? Does that mean if we get a THF electrical oscillator to emit the frequencies that are visible to the eye, do we see light coming out of the circuit?
 A: In undergraduate physics courses (c.f. MIT 8.03 course on waves), electromagnetic radiation is introduced as a pair of time-dependent oscillating magnetic and electric fields which satisfy the electromagnetic wave equation, as derived by Maxwell:
$$\frac{1}{c^2}\frac{\partial^2 E(x,t)}{\partial t^2} - \frac{\partial^2 E(x,t)}{\partial x^2} = 0$$
Electromagnetic radiation may also be viewed as a particle, i.e. a photon. In quantum electrodynamics, it arises as one quantizes classical electromagnetic field theory, given by the Maxwell Lagrangian,
$$\mathcal{L} = -\frac{1}{4}F_{\mu \nu}F^{\mu \nu}$$
The photon is a gauge boson which in cursory terms acts as a mediator of the electromagnetic force, just as the hypothesized graviton is a mediator of gravitation. The term 'light' refers to electromagnetic radiation, and hence photons. However, in the case of 'visible light,' this restricts us to only photons of a certain wavelength between 400 and 700 nanometers, which through the relation $E = hc/\lambda$ implies an energy range of approximately 1.7 to 3.1 electronvolts.  
